1. Field of the Invention
The present invention relates generally to apparatus for fabricating magnetic-recording and magneto-optical disks, and relates more specifically to a circularly symmetric, large-area, high-deposition-rate sputtering apparatus having a supported gas discharge provided by hollow-cathode plasma devices.
2. Description of the Relevant Art
In the fabrication of data-storage media (both magnetic-recording and magneto-optical) the most common technique for depositing the various thin-film layers is magnetron diode sputtering. The sputtering system approaches that are used in the production of magnetic-recording media consist mainly of two configurations: 1) for each thin-film layer, dual-side coating of a single disk substrate in the static-deposition mode from a pair of circularly symmetric planar (or planar ring) magnetron diode sputtering sources, and 2) for each thin-film layer, dual-side coating of a platen of several disk substrates in the dynamic deposition (passby) mode from a pair of rectangular planar magnetron diode sputtering sources.
Both approaches are best accomplished in systems where process-isolated chambers are provided so that preliminary steps of substrate outgassing, sputter-etch cleaning, and heating, during which reactive gases (water vapor, air, organic solvents) are evolved, do not interfere with later sputter-deposition steps, which take place in an inert gas atmosphere. Similarly, reactive sputter deposition, involving a target material where a reactive gas (or gases) is intentionally employed or involving a target material whose composition comprises a reactive-gas constituent or whose microstructure contains a reactive gas (or gases) in its micropores, must be carried out in an isolated process chamber. This extremely important aspect pertaining to vacuum cleanliness is much more easily implemented in a system design based upon static deposition than on one based upon passby deposition. The presence of unwanted reactive gases during sputtering has a deleterious effect on the morphology and the magnetic properties of the deposited films, greatly affecting their uniformity and reproducibility.
In the aforementioned static- and dynamic-deposition sputtering system approaches, due to the requirement of product throughput, the sequential steps are essentially simultaneous at several stations such that there is a disk substrate at or a platen of disk substrates passing by each of the several stations. The static-deposition approach, in which a single disk substrate per pair of circular symmetric sputtering sources is employed, has a significant advantage in that excellent circumferential uniformity of film morphology and thus of magnetic characteristics is obtained, even at the lowest (which is highly desirable) sputtering gas pressure. However, there are disadvantages to this particular static-deposition approach in that the product throughput and coating cost are independent of disk diameter and the economies of scale are not available for costly components, such as source power supplies and chamber vacuum valves, pumps, gauges, mass flow controllers, the transport mechanism, the process sequencer, etc.--the major cost-determining items of the sputtering system.
This situation is reversed in the passby approach where a platen of several disk substrates per pair of rectangular line-deposition sputtering sources is employed. However, a serious disadvantage entailed in the passby approach is the lack of circumferential uniformity of film morphology and hence of magnetic characteristics along the circular tracks of the disk. These undesirable features are caused by the source-determined differing and changing angles of incidence of the arriving sputtered atoms at the substrate as the platen passes by the line-deposition sources. This problem can be overcome by operation at higher sputtering gas pressure or with increased source-to-substrate separation or with some combination of both. Since the sputtered atoms undergo accordingly many more gas collisions in transit, their directionality is lost and their arrival angles become randomized, and thus the advantages of low-pressure deposition are thereby sacrificed. The once energetic sputtered atoms, losing their energy by gas collisional scattering, become thermalized. Consequently, the adhesion of the film to the substrate decreases, with an abrupt interfacial boundary forming instead of a graded diffused one. Additionally, the cohesive strength of the film decreases, with a resulting structure of thermalized-atom-deposited porous columnar Zone 1 or the still more porous Zone 1' (on the Movchan-Demchishin-Thornton zone-structure diagram) instead of the energetic-atom-deposited dense fibrous Zone T structure. References on the subject of coating zone structures include the following: J. A. Thornton, J. Vac. Sci. Technol. 11, 666 (1974); D. W. Hoffman and R. C. McCune in "Handbook of Plasma Processing Technology", S. M. Rossnagel, J. J. Cuomo, and W. D. Westwood, eds., Ch. 21, pp. 483-517, Noyes Publications, Park Ridge, N.J. (1990).
Another sputtering system approach, though not in general use for the production of data-storage media, entails the use of high-radiofrequency-powered (13.56 MHz) large-area circularly symmetric planar disk diode sputtering sources. In this approach, for each thin-film layer, dual-side coating of a platen of several disk substrates in the static-deposition mode from a pair of these large-area sputtering sources would be employed, thereby combining the advantages of the two aforementioned approaches with none of their respective disadvantages. There is, however, one inherent disadvantage of the 13.56 MHz rf planar disk diode sputtering source; namely, for a given system configuration, lower deposition rates are obtained as compared to magnetron diode sputtering sources. However, the sputtering-target utilization of rf planar disk diode sources is greater than 90%, which is very much better utilization than that of planar magnetron diode sources, particularly for target materials of ferromagnetic and ferrimagnetic substances.
In addition, a properly designed sputtering system configuration with two like oppositely facing rf planar disk diode sources requires that the chamber diameter or box size be at least three times the diameter of the source so that there be sufficient grounded area in contact with the gas discharge in order to keep the plasma potential low with respect to ground. Since such systems are geometry dependent with the rf power and hence voltage dividing according to the respective areas of the target electrode (or electrodes) and of the grounded walls in contact with the gas discharge (the Koenig-Maissel relationship), the plasma potential with respect to ground then (1) increases with increasing geometrical confinement for a given power input, (2) increases with increasing power input for a given pressure, and (3) decreases with increasing pressure for a given power input. Thus, inadvertently, surfaces other than the sputtering target become subject to energetic ion bombardment (i.e., sputtering) in systems with geometrically confined rf discharges, resulting in the contamination of the sputter-deposited films with the materials of the chamber construction. The significant references further describing this phenomenon are as follows: H. R. Koenig and L. I. Maissel, IBM J. Res. Develop. 14, 168 (1970); U.S. Pat. No. 3,661,761, invented by H. R. Koenig, issued May 9, 1972, assigned to IBM Corp.; J. W. Coburn and E. Kay, J. Appl. Phys. 43, 4965 (1972); J. L. Vossen, J. Electrochem. Soc. 126, 319 (1979); and H. R. Kaufman and S. M. Rossnagel, J. Vac. Sci. Technol. A6, 2572 (1988).
A method and device for obtaining a higher-plasma-density gas discharge by coupling a dc-powered hollow-cathode plasma device to a dc-powered planar disk (or rectangular) magnetron diode sputtering source, making in effect a triode configuration, are disclosed in J. J. Cuomo, H. R. Kaufman, and S. M. Rossnagel, U.S. Pat. No. 4,588,490, issued May 13, 1986 (filed May 22, 1985), assigned to IBM Corp. This hollow-cathode plasma device depends on thermionic electron emission for its operation.